Remove confusing plugin package, move to frontends.scalac

src/main/scala/leon/Main.scala

src/main/scala/leon/frontends/scalac/ASTExtractors.scala

src/main/scala/leon/frontends/scalac/CodeExtraction.scala

src/main/scala/leon/frontends/scalac/ExtractionPhase.scala

src/main/scala/leon/frontends/scalac/LeonExtraction.scala

src/main/scala/leon/frontends/scalac/ScalaCompiler.scala

src/main/scala/leon/frontends/scalac/SimpleReporter.scala

src/main/scala/leon/synthesis/utils/Benchmarks.scala

src/main/scala/leon/utils/TemporaryInputPhase.scala